For logistical reasons maybe they didn't want to bring in all of their troops into the fight? Every new soldier and vehicle you send into a battle is an asset that will eventually need more ammo, food or fuel in the case of BMPs and Tanks. The push from Belarus down into Kiev makes sense because they are effectively trying to cut off the head of the Ukrainian government. Advances from the east could also eventually try to go for Kiev. The troops trying to move up from Crimea could be trying to secure the
North Crimean Canal, beyond that I have no clue what the objectives could be on that front.